This invention relates to certain high strength and high modulus polypyromellitimide filaments and to a method of obtaining them. Aromatic polyimides, and particularly polypyromellitimides, are well known for their high temperature physical and chemical stability. Certain polyimides or modified polyimides have been commercialized as high temperature fibers but their tenacity or modulus are inadequate for reinforcement purposes.
U.S. Pat. No. 3,415,782 discloses a process whereby a polyamide-acid is formed into a shaped article and the article then treated to convert the polyamide-acid polymer to the polyimide thereof. The polyimide polymers described therein were prepared from pyromellitic dianhydride and bis(4-aminophenyl)ether or bis(4-aminophenyl)sulfide.
In the case of filaments comprising polyimide polymers of the aforementioned U.S. Pat. No. 3,415,782, it has been found that in some applications these filaments have insufficiently high modulus for applications in plastics reinforcement where carbon, glass and Kevlar.RTM. aramid fibers are currently used.